Within the context of the present disclosure, the expression “internal combustion engine” encompasses Otto-cycle engines but also diesel engines and hybrid internal combustion engines, which utilize a hybrid combustion process, and also hybrid drives which comprise not only the internal combustion engine but also an electric machine which can be connected in terms of drive to the internal combustion engine and which receives power from the internal combustion engine or which, as a switchable auxiliary drive, additionally outputs power.
Internal combustion engines have a cylinder block and at least one cylinder head which are connected to one another at an assembly end side to form the cylinders. To control the charge exchange, an internal combustion engine requires control elements—generally in the form of lifting valves—and actuating devices for actuating these control elements. The valve actuating mechanism required for the movement of the valves, including the valves themselves, is referred to as the valve drive. The cylinder head often serves to accommodate the valve drive.
During the charge exchange, the combustion gases are discharged via the outlet openings of the cylinders and the charging with charge air takes place via the inlet openings. It is the object of the valve drive to open and close the inlet and outlet openings at the correct times, with a fast opening of the largest possible flow cross sections being sought in order to keep the throttling losses in the inflowing and outflowing gas flows low and in order to ensure the best possible charging and an effective, that is to say complete, discharge of the exhaust gases. According to the prior art, therefore, the cylinders are also often provided with two or more inlet and outlet openings.
According to the prior art, the exhaust lines which adjoin the outlet openings are at least partially integrated in the cylinder head. The exhaust lines of the cylinders are generally merged to form one common overall exhaust line, or else in groups to form two or more overall exhaust lines. The merging of exhaust lines to form an overall exhaust line is referred to generally, and within the context of the present disclosure, as an exhaust manifold.
Downstream of the manifolds, the exhaust gases are may be supplied, for the purpose of supercharging the internal combustion engine, to the turbines of at least two exhaust-gas turbochargers. The advantages of an exhaust-gas turbocharger for example in relation to a mechanical charger are that no mechanical connection for transmitting power exists or is required between the charger and internal combustion engine. While a mechanical charger extracts the energy required for driving it entirely from the internal combustion engine, and thereby reduces the output power and consequently adversely affects the efficiency, the exhaust-gas turbocharger utilizes the exhaust-gas energy of the hot exhaust gases. An exhaust-gas turbocharger arranged in the exhaust-gas discharge system however results in increased exhaust-gas back pressure upstream of the turbine, which can have an adverse effect on the charge exchange.
An exhaust-gas turbocharger comprises a compressor and a turbine which are arranged on the same shaft. The hot exhaust-gas flow is supplied to the turbine of the charger and expands in said turbine with a release of energy. The shaft is thus set in rotation. The energy supplied by the exhaust-gas flow to the turbine and ultimately to the shaft is used for driving the compressor which is likewise arranged on the shaft. The compressor delivers and compresses the charge air supplied to it, as a result of which supercharging of the at least two cylinders is obtained. A charge-air cooling arrangement may be provided, by means of which the compressed charge air is cooled before it enters the cylinders.
Supercharging serves primarily to increase the power of the internal combustion engine. Here, the air required for the combustion process is compressed, as a result of which a greater air mass can be supplied to each cylinder per working cycle. In this way, the fuel mass and therefore the mean pressure can be increased. Supercharging is a suitable means for increasing the power of an internal combustion engine while maintaining an unchanged swept volume, or for reducing the swept volume while maintaining the same power. In any case, supercharging leads to an increase in volumetric power output and an improved power-to-weight ratio. For the same vehicle boundary conditions, it is thus possible to shift the load collective toward higher loads, at which the specific fuel consumption is lower.
The configuration of the exhaust-gas turbocharging often poses difficulties, wherein it is basically sought to obtain a noticeable performance increase in all rotational speed ranges. According to the prior art, a severe torque drop is however observed in the event of a certain engine speed being undershot. Said torque drop is understandable if one takes into consideration that the charge pressure ratio is dependent on the turbine pressure ratio. For example, if the engine speed is reduced, this leads to a smaller exhaust-gas mass flow and therefore to a lower turbine pressure ratio. This has the result that, toward lower engine speeds, the charge pressure ratio likewise decreases, which equates to a torque drop.
Here, it would fundamentally be possible for the drop in charge pressure to be counteracted by means of a reduction in the size of the turbine cross section, and the associated increase in the turbine pressure ratio. This however merely shifts the torque drop further in the direction of lower engine speeds. Furthermore, said approach, that is to say the reduction in size of the turbine cross section, is subject to limits because the desired supercharging and performance increase should be possible even at high engine speeds or in the case of large exhaust-gas flow rates.
In the prior art, it is sought, using a variety of measures, to improve the torque characteristic of a supercharged internal combustion engine.
This is achieved for example by means of a small design of the turbine cross section and simultaneous provision of an exhaust-gas blow-off facility. Such a turbine is also referred to as a wastegate turbine. If the exhaust-gas mass flow exceeds a critical value, a part of the exhaust-gas flow is, within the course of the so-called exhaust-gas blow-off, conducted via a bypass line past the turbine. Said approach has the disadvantage that the supercharging behavior is inadequate at relatively high engine speeds or in the case of relatively large exhaust-gas flow rates. Furthermore, according to the prior art, the blown-off exhaust gas is conducted past the turbine without being used further, and without the energy available in the hot exhaust gas being utilized.
The torque characteristic of a supercharged internal combustion engine may furthermore be improved by means of multiple turbochargers arranged in parallel, that is to say by means of multiple turbines of relatively small turbine cross section arranged in parallel, wherein turbines are activated successively with increasing engine speed or increasing exhaust-gas flow rate, in accordance with so-called sequential supercharging.
The inventors herein have recognized issues with the above-described engine systems. For example, internal combustion engines of said type still have potential for improvement because, according to the prior art, the second turbine of the second exhaust-gas turbocharger, said second turbine being acted on constantly with hot exhaust gas when the internal combustion engine is in operation, is equipped with a bypass line and exhaust gas is blown off in order to limit the charge pressure, adhere to the choke limit of the turbine and prevent thermal overloading of the turbine. According to the prior art, the blown-off exhaust gas is blown off without the energy available in the hot exhaust gas being utilized. If it were possible, by contrast, for the energy to be utilized, it would be possible to further increase the overall efficiency of the internal combustion engine.
If the first turbine is, according to the prior art, in the form of activatable turbine, further disadvantages arise. The rotational speed of the turbine decreases drastically when the latter is deactivated, such that, upon reactivation, the rotor of said turbine must initially be accelerated in order to be able to generate and provide the desired charge pressure at the compressor side. The response behavior is consequently impaired.
To ensure a minimum rotational speed of the activatable turbine, the activatable turbine could be supplied with a small exhaust-gas flow even when its outlet openings are deactivated. For this purpose, it would be necessary for a corresponding line to connect the second exhaust manifold to the first turbine, possibly with the use of at least one additional shut-off element, though this would disadvantageously increase the complexity and space requirement of the exhaust line system upstream of the turbines.
A further disadvantage of the use of an activatable turbine of the type described above, in which activatable outlet openings are used as switching means, consists in that, upon the activation of the outlet openings for the purposes of activating the turbine, the exhaust-gas flow conducted through the second turbine abruptly decreases, as in each cylinder there is now a further outlet opening, specifically the activated outlet opening, available for the exhaust gas during the charge exchange. Upon the activation of the first turbine, the charge pressure generated by the second compressor then abruptly decreases. The torque drop associated with the drop in charge pressure is undesirable.
Accordingly, examples are provided herein to at least partly address the above issues in order to improve the transient behavior of the exhaust-gas turbocharging arrangement. One example method includes, responsive to a first condition, deactivating a first compressor of a first turbocharger, activating each first exhaust valve of each cylinder of an engine, and deactivating each second exhaust valve of each cylinder of the engine to flow exhaust gas from the engine to a second turbocharger. The method further includes, responsive to boost pressure exceeding a threshold, maintaining deactivation of the first compressor, reactivating each second exhaust valve to flow exhaust gas from the engine to both the first turbocharger and second turbocharger, and driving an electric assist device via a first turbine of the first turbocharger.
In this way, during a first condition where boost demand may be met with a single turbocharger, the additional turbocharger may be deactivated and all exhaust from the engine may flow only to the single turbocharger, thus improving the transient response of the engine. If the boost pressure provided by the single turbocharger is greater than a boost demand, for example, or reaches high enough levels to cause turbocharger choke, excess exhaust gas may be diverted to the additional turbocharger rather than blown off via a wastegate or bypass. This excess exhaust gas may then be used to drive an electric assist device via the additional turbocharger. During other conditions (e.g., high engine speed/load conditions), both turbochargers may be active and provide boost to meet the higher torque demand.
By doing so, the presence of a wastegate or turbine bypass may be eliminated, thus lowering the cost, complexity, and packaging space of the engine, while still providing sufficient boost control. Further, by directing the excess exhaust gas to the first turbine via control of exhaust valves rather than a communication valve or other mechanism, exposure of such control mechanisms to high temperature/pressure exhaust may be avoided, prolonging the life of the system and lowering costs.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.